1. Field of the Invention
The present invention relates to an image forming apparatus in which toner images formed on an image bearing member are transferred onto an intermediate transfer member, and then the toner images on the intermediate transfer member are transferred onto a transfer material.
2. Related Background Art
In four full-color image forming apparatuses using an intermediate transfer member, firstly-transferring process for transferring a toner image formed on an image bearing member (for example, a photosensitive drum) onto an intermediate transfer member (for example, an intermediate transfer drum or an intermediate transfer belt) is repeated for four color toners to successively transfer the four color toner images onto the intermediate transfer member in a superimposed fashion. Then, secondary-transferring process for collectively transferring the four color toner image on the intermediate transfer member onto a transfer material such as a paper sheet is effected to obtain a four full-color image.
After the secondary-transferring process, toner which was not secondary-transferred to the transfer material (referred to as "secondary-transferring residual toner" hereinafter) is remaining on the surface of the intermediate transfer member by several percentage (%). If the secondary-transferring residual toner leaves as it is, during next image formation, the secondary-transferring residual toner will be transferred onto a next transfer material to result in poor image, or will be scattered within the image forming apparatus to contaminate the transfer material. Thus, in the image forming apparatuses using the intermediate transfer member, there is a problem how to remove or clean the secondary-transferring residual toner remaining on the intermediate transfer member.
To solve this problem, for example, as disclosed in U.S. Pat. No. 5,732,310, there has been proposed a technique in which the secondary-transferring residual toner on the intermediate transfer member is charged to polarity (positive) opposite to normal polarity (negative) of the toner on the photosensitive drum by using a charge roller, and, at a firstly transfer station, the secondary-transferring residual toner is reversely-transferred from the intermediate transfer member onto the photosensitive drum to collect the residual toner by utilizing difference in potential (electric field) between surface potential (firstly-transfer bias) of the intermediate transfer member and surface potential of the photosensitive drum. In this case, the reverse transferring process for transferring the secondary-transferring residual toner from the intermediate transfer member onto the photosensitive drum and the firstly-transferring process for transferring the toner image from the photosensitive drum onto the intermediate transfer member are effected simultaneously to improve through-put of the image formation.
However, in the above-mentioned image forming apparatus, there arose the following problem.
In the reverse transferring process for transferring the secondary-transferring residual toner from the intermediate transfer member onto the photosensitive drum, the secondary-transferring residual toner is shifted by the difference in potential (electric field) between the surface potential (firstly-transfer bias) of the intermediate transfer member and the surface potential of the photosensitive drum. For example, when the surface of the photosensitive drum is charged to -550 V by a first charger and the firstly-transfer bias to be applied to the intermediate transfer member is set to +200 V, due to the difference in potential (electric field) of 750V therebetween, the toner having reverse polarity (positive polarity) is collected onto the photosensitive drum.
By the way, in full-color image formation requiring high density stability, in order to cope with time-lapse degradation of the photosensitive drum, there is provided a density detect means for detecting density of the toner image on the photosensitive drum, and automatic control is effected on the basis of the detected result to optimize the density of the toner image. As methods for controlling the density, there have been proposed a method for controlling a toner fog condition by changing the charged potential of the photosensitive drum and a method for controlling the optimum density by changing developing bias.
For example, when the surface potential of the photosensitive drum is set to -650 V, due to the potential control for the photosensitive drum, the difference in potential between the surface potential and the firstly-transfer bias becomes 850 V. By the way, if the difference in potential becomes greater than 800 V, at a firstly transfer nip between the photosensitive drum and the intermediate transfer member, Paschen discharge is generated, so that the secondary-transferring residual toner which was charged positively by the charge roller is charged negatively (minus) again, thereby causing poor cleaning (in which the residual toner is not collected onto the photosensitive drum). If such poor cleaning is caused, the secondary-transferring residual toner is accumulated on the intermediate transfer member, so that the residual toner is adhered to a succeeding (next) transfer material or is scattered within the image forming apparatus, thereby causing poor image.